There are not many comparable to John Tavares in the post-cap era. Here are the best players I know of who have been traded, I list some of the differences and comment on the assets:
Rick Nash+Steven Delisle+Conditional 2013 3rd rounder
for
Artem Anisimov, Brandon Dubsinky, Tim Erixon, 2013 1st rounder (Kerby Rychel)
Rick Nash was 28 years old at the time of the trade and he had five years remaining on contract, at a cap hit paying 7.8 million.
2012 off-season
Analysis: Nash is a lesser player than Tavares, though he was in a more valuable contract situation, and he was traded for 4 pieces of intermediate value. This was the best deal Columbus could get after advertising their sale to everybody for a period of 1 year.
Chris Pronger+Ryan Dingle
for
Joffrey Lupul, Luca Sbisa, two 1st rounders (Emerson Etem, Peter Holland)
Following which, Pronger signed a 7-year contract extension
2009 off season
Analysis: This trade didn't work out but it made a lot of sense. Lupul is an intermediate-value piece just like what the Blue Jackets got for Nash, but the Flyers gave two 1st rounders rather than one. Further, Luca Sbisa was then viewed as an elite defensive prospect. He's turned out to be merely ok, but at the time his potential was regarded as extremely high.
Marian Hossa+Pascal Dupuis
for
Eric Christensen, Colby Armstrong, Angelo Esposito, 1st round pick (Daultan Leveille)
Contract: This was a deadline rental, which may explain why Atlanta got the least value out of the trades discussed in this post.
Analysis: Atlanta got two intermediate-value pieces in Christensen and Armstrong (who then declined), Esposito who was believed to be an elite prospect, and a late 1st rounder.
Global Analysis:
The typical package is a late 1st rounder, a good-but-not-elite prospect, and some intermediate-value roster players.
There are no Pacioretty's or Gallagher's in these trades, only intermediate-value roster players are trade, which on the Habs might mean Nathan Beaulieu, Greg Pateryn.
Sbisa is the only plausible comparable to Sergachev. Most of the traded prospects were not great. There are however several players comparable to McCarron, Scherbak, and Juulsen, typically 1 per trade.
In spite of the fact that some here think Timmins should be fired for not batting 1.000 in the first round, none of the 1st rounders resulted in home runs. Life goes on without Kerby Rychel, Emerson Etem, Peter Holland, and Daultan Leveille.
There are no cap dumps in these trades, but I think that's a fluke. There can be cap dumps, particularly with the changes to the CBA formula after the last lockout.
So, a likely traded for Tavares given the historical model is Cap dump, Beaulieu, McCarron, and a 2017 1st rounder. Islanders might also throw in a 5th rounder, because these marginal value throw-ins seem to make it into these trades.